The endogenous sens locus has been mutated so that it is flanked on one side by an attB site and on the other by an attP site. In addition, a loxP cassette containing the w+mW.hs marker is present, downstream of the sens gene but upstream of the attP site. The presence of the flanking attP and attB sites results in a conditional null allele in which phiC31:int integrase can be used to excise the sens locus plus w+mW.hs marker cassette.